In household electrical appliances, building materials, automobiles, etc., instead of the conventional post-coated products which are coated after being formed, pre-coated steel plates coated by pre-colored organic coating films have come to be used. These pre-coated steel plates are comprised of steel sheets treated to prevent rust or plated steel sheets which are then coated by colored organic coating films, so have beauty, yet have formability and are excellent in corrosion resistance.
For example, PLT 1 discloses the art of defining the structure of the coating film so as to obtain a pre-coated steel plate excellent in formability, contamination resistance, and hardness. On the other hand, PLT 2 discloses a pre-coated steel plate using a specific chromate treatment solution so as to improve the corrosion resistance at the end faces. These pre-coated steel plates have as their object to provide good corrosion resistance and also formability and coating adhesion by the composite effect of the plating layer, chromate treated films, and the primer films containing a chromic rust-preventing pigment, to eliminate the coating after forming, and to improve the productivity and quality.
However, considering the environmental load of the hexavalent chromium which may be eluted from the chromate treated films and organic coating films containing chromium-based rust-preventing pigments, recently demand has been growing for non-chromium rust prevention and non-chromium organic coating films. To deal with this, for example, PLT 3 and PLT 4 disclose non-chromium-based pre-coated steel plates excellent in corrosion resistance. These have already been commercialized.
The coatings used for these pre-coated steel plates give thick coatings of 10 μm or more. In addition, since a large amount of solvent-based paint is used, incinerators, facilities for dealing with odors, and other specialized coating facilities are required. Production on specialized coating lines is the general practice. That is, an extra coating step is involved in addition to the steps of production of steel plate for coating, so large expenses are required in addition to the costs of the materials required for the coating. Therefore, the obtained pre-coated steel plates are expensive.
However, due to increasingly diverse user needs, there is a demand for colored steel plates in household electrical appliances, interior building materials, or other fields in which the objective is sufficiently achieved if having durability under routine usage conditions. Lower priced products are being sought. That is, conventional expensive pre-coated steel plates alone are not sufficient for meeting the increasingly diverse demand.
To deal with these needs, as colored steel plates able to be inexpensively produced, for example, PLT 5 discloses a colored steel plate provided with a colored resin layer of a thickness of 5 μm or less, while PLT 6 discloses a colored steel plate having a color forming coating film on the surface of steel having a specific roughness. However, these colored steel plates are designed to secure corrosion resistance by chromate treated coating films, so cannot handle the recent needs for non-chromium use. In addition, these are not designed considering the concealing power at locations which are worked and the colored layers are stretched, so there was also the problem of a remarkable decreasing in the appearance of the worked parts.